Optical communication systems which operate in the wavelength range from 1100 to 1700 nanometers (nm) are of potentially great importance because the dispersion and losses in an optical fiber are typically very low in this wavelength range. Heterojunction devices incorporating binary III-V alloys and solid solutions of these alloys have been found to be particularly useful for this application because their electronic bandgaps occur in this wavelength range and lattice-matched heterojunctions can be obtained by compositional variations. In particular, ternary and quaternary alloys of In, Ga, As and P on an InP substrate have been found to be useful materials for both light-emitters and detectors.
Problems which have affected the performance of avalanche photodetectors using these materials include bulk tunneling currents which occur at electric fields of the order of 1.5.times.10.sup.5 V/cm in the ternary and quaternary compounds used for the light-absorptive region, edge breakdown and multiplication of surface leakage currents at the junction periphery. The tunneling has been reduced by locating the P-N junction with its high electric field in a wide bandgap material separated from the light-absorptive region in the narrower bandgap material. This is the so-called SAM (Separated Absorbing and Multiplying) avalanche photodetector structure.
Edge breakdown and surface currents have been reduced by the use of surface contouring of the detector sidewalls. However, the electrical field reduction at the surface may be small with the result that the surface dark current may still undergo multiplication. Therefore, it is desirable to further reduce the electric field at the junction periphery to further limit the surface electric field and current.